Documentation/filesystems/inotify.txt

   1                                    inotify
   2             a powerful yet simple file change notification system
   3
   4
   5
   6 Document started 15 Mar 2005 by Robert Love <rml@novell.com>
   7 Document updated 4 Jan 2015 by Zhang Zhen <zhenzhang.zhang@huawei.com>
   8         --Deleted obsoleted interface, just refer to manpages for user interface.
   9
  10 (i) Rationale
  11
  12 Q: What is the design decision behind not tying the watch to the open fd of
  13    the watched object?
  14
  15 A: Watches are associated with an open inotify device, not an open file.
  16    This solves the primary problem with dnotify: keeping the file open pins
  17    the file and thus, worse, pins the mount.  Dnotify is therefore infeasible
  18    for use on a desktop system with removable media as the media cannot be
  19    unmounted.  Watching a file should not require that it be open.
  20
  21 Q: What is the design decision behind using an-fd-per-instance as opposed to
  22    an fd-per-watch?
  23
  24 A: An fd-per-watch quickly consumes more file descriptors than are allowed,
  25    more fd's than are feasible to manage, and more fd's than are optimally
  26    select()-able.  Yes, root can bump the per-process fd limit and yes, users
  27    can use epoll, but requiring both is a silly and extraneous requirement.
  28    A watch consumes less memory than an open file, separating the number
  29    spaces is thus sensible.  The current design is what user-space developers
  30    want: Users initialize inotify, once, and add n watches, requiring but one
  31    fd and no twiddling with fd limits.  Initializing an inotify instance two
  32    thousand times is silly.  If we can implement user-space's preferences
  33    cleanly--and we can, the idr layer makes stuff like this trivial--then we
  34    should.
  35
  36    There are other good arguments.  With a single fd, there is a single
  37    item to block on, which is mapped to a single queue of events.  The single
  38    fd returns all watch events and also any potential out-of-band data.  If
  39    every fd was a separate watch,
  40
  41    - There would be no way to get event ordering.  Events on file foo and
  42      file bar would pop poll() on both fd's, but there would be no way to tell
  43      which happened first.  A single queue trivially gives you ordering.  Such
  44      ordering is crucial to existing applications such as Beagle.  Imagine
  45      "mv a b ; mv b a" events without ordering.
  46
  47    - We'd have to maintain n fd's and n internal queues with state,
  48      versus just one.  It is a lot messier in the kernel.  A single, linear
  49      queue is the data structure that makes sense.
  50
  51    - User-space developers prefer the current API.  The Beagle guys, for
  52      example, love it.  Trust me, I asked.  It is not a surprise: Who'd want
  53      to manage and block on 1000 fd's via select?
  54
  55    - No way to get out of band data.
  56
  57    - 1024 is still too low.  ;-)
  58
  59    When you talk about designing a file change notification system that
  60    scales to 1000s of directories, juggling 1000s of fd's just does not seem
  61    the right interface.  It is too heavy.
  62
  63    Additionally, it _is_ possible to  more than one instance  and
  64    juggle more than one queue and thus more than one associated fd.  There
  65    need not be a one-fd-per-process mapping; it is one-fd-per-queue and a
  66    process can easily want more than one queue.
  67
  68 Q: Why the system call approach?
  69
  70 A: The poor user-space interface is the second biggest problem with dnotify.
  71    Signals are a terrible, terrible interface for file notification.  Or for
  72    anything, for that matter.  The ideal solution, from all perspectives, is a
  73    file descriptor-based one that allows basic file I/O and poll/select.
  74    Obtaining the fd and managing the watches could have been done either via a
  75    device file or a family of new system calls.  We decided to implement a
  76    family of system calls because that is the preferred approach for new kernel
  77    interfaces.  The only real difference was whether we wanted to use open(2)
  78    and ioctl(2) or a couple of new system calls.  System calls beat ioctls.
  79